U.S. Rep. Kevin Cramer, R-N.D., running for U.S. Senate against Sen. Heidi Heitkamp, D-N.D., said in a statement Pruitt "sought balance in carrying out the mission of environmental protection while looking out for our economy and job creation."

Cramer commended Pruitt for focusing on the EPA's "original purpose under cooperative federalism," adding in a later interview he meant the collaboration between state and federal government, instead of the federal government imposing "one size fits all" rules on states.

He didn't address agency investigations on Pruitt's spending and lobbying—instead, he ended his statement endorsing Andrew Wheeler, Pruitt's deputy administer, who will lead the EPA until the Senate votes on an official successor for Pruitt.

Wheeler will start Monday, according to a tweet from the president.

Heitkamp shared similar optimism for Wheeler, saying in a statement Friday morning she met with him several months ago, emphasizing the need for a strong Renewable Fuel Standard.

The RFS requires businesses to mix more renewable fuel with transportation fuel every year, with goals of reaching 36 billion mixed gallons by 2022.

Heitkamp was one of two Democratic senators who voted for Pruitt last year, and she was one of 10 who signed a bipartisan letter to Pruitt in March, urging him not to undermine the RFS. "The RFS is an effective driver of economic development," the letter said. "It has strengthened markets and created hundreds of thousands of jobs in the new energy economy, and driven economic growth in rural communities across the country."

Heitkamp signed another letter to Pruitt in April, talking about what she said was Pruitt's lack of support for ethanol.

"Scott Pruitt said he would support the Renewable Fuel Standard, but that clearly didn't hold water, and I continued to hold his feet to the fire on that issue," Heitkamp said.

Before voting for Pruitt last February, Heitkamp released a similar statement, in which she mentioned talking to Pruitt about supporting a comprehensive energy plan.

"That means supporting fossil fuels, as well as renewable energies and biofuels—and we can do that while reducing emissions to make sure we have clean air and water," Heitkamp said.

Sen. John Hoeven, R-N.D., said in a statement Friday he appreciated Scott Pruitt's policy work, but recognized "there are other issues in regard to ethics investigations and concerns that resulted in his resignation." The investigations Hoeven mentioned include reports of Pruitt using federal resources to find his wife a job, spending thousands of unnecessary dollars on home and office furnishings and making energy decisions based on discussions with organizations in the energy industry.

Sen. Tina Smith, D-Minn., tweeted she was glad Pruitt stepped down after she had "repeatedly called on him to do so."

"The Trump administration needs to replace him with somebody who has the integrity and effectiveness to fight for our environment," she said.

Sen. Amy Klobuchar, D-Minn., released a briefer statement on Twitter, sharing a New York Times article on Pruitt's resignation and scandals. "Finally," she said.